The R and the M denote Automatic Radiator and Automatic Mixture respectively. As you might imagine they automatically control the radiator cooling (by opening / closing the radiator vents as needed) and automatically adjusting fuel mixture which should be full (or near full) rich at ground level and be gradually leaned out as you gain altitude.

Generally, most servers (particularly expert servers like our Wargrounds or Cuban's VM Syndicate) expect you to manage these manually, but in icons servers, or our Basic Training server and SP missions, they may be on to make it easier for new pilots to start and fly their planes.

If they are available to you, the default keys to toggle them on/off are Left-Shift R and Left Shift M.

Another one you'll see is an H, which is for Altitude Hold (ie. autolevel). That is handy for longer cruise flights, or if you are flying through clouds. You can toggle that one with Left-Shift A.